A study dealing with the ground state and excited state behavior of arylazirines and 1,3-diazabicyclo(3.1.0)hex-3-enes has been initiated. The results obtained indicate that the photolysis of azirines proceeds by way of irreversible opening of the three-membered ring to form a nitrile ylide intermediate. The transient nitrile ylide undergoes subsequent 1,3-dipolar addition onto the azirine double bond to give the 1,3-diazabicyclo(3.1.0)hex-3-ene system. On further irradiation, the 1,3-diazabicyclic system is converted into a substituted dihydropyrazine. This latter transformation proceeds by ring opening to an enedimine which thermally cyclizes to a dihydropyrazine. Trapping experiments indicate that the photo-induced ring opening reaction proceeds by a disallowed conrotatory rotation. A similar disallowed valence tautomerism of a diazabenzobicyclo(3.1.0)hexene system was also encountered in our studies. Finally, the preparation and chemistry of the tricyclo(3.2.0.02,6)heptan-7-o1 system was examined.